1. Field of the Invention
This invention relates to a vacuum booster comprising a booster shell comprised of a front shell half and a rear shell half interconnected with their open ends opposed to each other, and a diaphragm which divides an interior of the booster shell into a vacuum chamber on a side of the front shell half and a working chamber on a side of the rear shell half and is provided with an outer peripheral bead portion clamped between the open ends of the shell halves with an outer surface of the outer peripheral bead portion being contacted under pressure with an inner surface of the open end of the front shell half.
2. Description of the Prior Art
Such a vacuum booster is already known, for example, from Japanese Utility Model Application Laid-open No. 108857/88.
In order to insure a sufficient airtightness of the vacuum chamber, it is desirable to establish a large surface pressure between the outer surface of the outer peripheral bead portion of the diaphragm and the inner surface of the open end of the front shell half. In the above-described prior art vacuum booster, however, the outer peripheral bead portion is formed into a mere cylindrical shape with its wall thickness being substantially constant. To increase the surface pressure, it is necessary to provide a relatively large wall thickness of the outer peripheral bead portion in its natural state, permitting the outer peripheral bead portion to be deflected in an increased amount in its applied state. However, the increase in wall thickness in such a manner results in a large frictional resistance when the outer peripheral bead portion of the diaphragm is inserted into the open end of the front shell half during assembling, leading to an inferior assembling.